1. Field of the Invention
The present invention relates to a magnetic recording medium such as a magnetic tape, a magnetic disc and a magnetic card and more specifically to an improvement whereby an electrically conductive primer coating is formed between a non-magnetic substrate and a magnetic layer.
2. Description of the Related Art
In general, a magnetic recording medium comprises a non-magnetic substrate, such as a polyester film, and a magnetic layer formed by applying a magnetic paint comprising a magnetic powder, a binder resin, a dispersant, and a lubricant in an organic solvent on the substrate and drying it.
When the magnetic layer has a large surface resistance, it is easily charged with electricity which may cause drop out, due to attracted dust or noise, due to discharge during recording and reproducing.
To form a magnetic layer having a low surface resistance, an electrically conductive material, such as carbon black, is added to the magnetic layer.
Recently, as the recording density of the magnetic recording medium is increased, it has been attempted to increase the amount of the magnetic powder in the magnetic layer, to improve surface smoothness of the magnetic layer and to decrease the thickness of the magnetic layer.
To decrease the surface resistance of the magnetic layer, it has be contemplated to increase the amount of an electrically conductive material, such as carbon black, in the magnetic layer. However, since carbon black is non-magnetic and has a small specific gravity, increase of the amount of carbon black decreases the relative amount of the magnetic powder.
Heretofore, to remove non-magnetic carbon black having a small specific gravity from the magnetic layer, it has been proposed to form a separate primer coating comprising the electrically conductive material, such as carbon black, and a binder resin so as to impart the conductivity to the magnetic recording medium (cf. Japanese Patent Kokai Publication Nos. 150129/1983 and 20316/1985).
To form such a primer coating, a paint comprising the binder resin and an electrically conductive material homogeneously dispersed in the resin is prepared and applied on the non-magnetic substrate. However, the homogeneous dispersion of the electrically conductive material in the binder resin is very troublesome and further the paint cannot be stored for a long time while maintaining the dispersed state of the electrically conductive material. Since the surface smoothness of the primer coating depends on the dispersed state of particles of the electrically conductive material, it is very difficult to produce the magnetic recording medium having good surface smoothness even if the non-magnetic substrate has good surface smoothness. Therefore, the requirement for improved surface smoothness of the magnetic recording medium cannot be fulfilled.